In modern commercial aircraft heat exchangers are installed in various systems, such as for example an aircraft air conditioning system, a de-icing system or cooling system for cooling thermally loaded equipment on board the aircraft. Depending on requirements, the heat exchangers may take the form of gas-gas heat exchangers, gas-liquid heat exchangers or liquid-liquid heat exchangers. Heat exchangers that are used to cool hot bleed air removed from the engine compressors or auxiliary engine compressors for further use in the aircraft air conditioning system or the de-icing system of the aircraft are currently cooled by engine cooling air removed from the engine, so-called “fan air”. Alternatively, the heat exchangers, through which hot engine bleed air flows, may also be cooled by ambient air that flows through a cooling air duct. In order to convey the ambient air through the cooling air duct, hot engine bleed air may be directed via an injector nozzle into the cooling air duct. The jet pump effect produced by the injection of engine bleed air into the cooling air duct then ensures that sufficient ambient air is sucked into the cooling air duct and through the heat exchanger that is to be cooled.
Particularly in aircraft that are equipped with propeller engines there is the problem that no engine cooling air may be removed from the engines. The cooling of a heat exchanger, through which hot engine bleed air flows, then inevitably requires the use of a cooling duct, into which hot engine bleed air is injected through an injector nozzle in order to convey ambient air through the cooling air duct and through the heat exchanger that is to be cooled. The injection of highly compressed engine bleed air into a cooling duct may however lead to considerable noise emissions because of the expansion of the engine bleed air in the cooling duct. Furthermore, the removal of large quantities of heat from the heat exchanger, through which hot engine bleed air flows, requires a high cooling air-mass flow, which may be conveyed through the cooling duct only by the injection of a correspondingly high injection air-mass flow into the cooling duct. Finally, a cooling system, in which ambient air is conveyed through a cooling duct by the injection of engine bleed air into the cooling duct, has the drawback that the engine bleed air is utilized, not directly, but only indirectly for cooling purposes. This leads to losses in the energy efficiency of the system.
The invention is geared to the object of indicating a system for cooling a heat exchanger on board an aircraft that may be operated in an energy-efficient manner without the use of engine cooling air.